While manufacturing semiconductor devices such as FinFETs having Lightly Doped Drain (LDD) structures, a nitride seal, such as a silicon nitride (SiN) layer, serves as a diffusion barrier to oxygen and prevents contamination during subsequent processing. However, the thickness of the SiN layer may become thinner than desired during a LDD wet cleaning process. It would worsen the performance of the semiconductor device and causes a serious dopant-loss problem, and might cause bump issues after an epitaxial (EPI) forming process.